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  • A-Level Physics

Capacitors

Capacitors store electrical charge. They are made up of two conducting plates separated by a dielectric (insulator) or gap. A positive and a negative charge builds up on the opposite plates, but the insulator stops the charge from moving – this creates a potential difference via a uniform electric field. The charges on each plate are equal and opposite.

Capacitor diagram, capacitance diagram, dielectric diagram. EngineeringNotes.net, EngineeringNotes, Engineering Notes

Capacitance is the of a capacitor is defined as the charge stored per unit p.d. across the capacitor.

C =Q/V capacitance = charge / potential difference

The units of capacitance are Farads, F, or Coulomb per Volt, C/V.




Capacitor Networks

As a general rule of thumb, everything about capacitors is the opposite to how it would be with resistors:

Capacitor network diagram, capacitors in series and parallel, capacitor circuit diagram, capacitor symbol, capacitor diagram, capacitance. EngineeringNotes.net, EngineeringNotes, Engineering Notes
1/C = 1/C(1) + 1/C(2) … Combined Capacitance in series
C = C(1) + C(2) … Combined Capacitance in parallel



Energy Stored

Work is done removing negative charge from one plate and depositing it on the other plate – this is the same as the energy stored by the capacitor, and can be found as the area under a p.d. – charge graph:

Capactitance graph, energy stored graph, capacitor energy graph, p.d. - charge graph capacitors, voltage - charge graph capacitor. EngineeringNotes.net, EngineeringNotes, Engineering Notes
W = 1/2 QV Energy stored = 1/2 x charge x p.d.

This equation can be rewritten using the capacitance equation, C = Q/V, rearranged as Q = CV

W = 1/2 V²C Energy Stored = 1/2 voltage² x capacitance

Or, using the capacitance equation written in the form V = Q/C: